It is frequently necessary to effect a seal in a clearance gap between two components that are capable of relative movement. In particular seals are often required to seal between a rotatable shaft and a housing surrounding the shaft, the housing having a bore through which the shaft extends.
Within a gas turbine engine there are shafts that rotate at relatively high speeds and are exposed to pressurised hot gases. Seals are required for these shafts and the seal performance can have a significant effect on the overall efficiency of the gas turbine engine. There are a number of seal designs that have been proposed for such purposes and that are designed to operate within the harsh environment found within a gas turbine engine.
One type of seal that has been developed for shafts that rotate at relatively high speeds, as found in gas turbine engines, is a brush seal. In such a brush seal, a plurality of fine bristles (such as bronze or stainless steel wire) are held in a carrier mounted on a housing. The tips of the bristles wipe against the shaft so as to effect a seal thereagainst.
A problem with such brush seals is that in use the bristles tend to flex in an axial, sealing direction, away from the high pressure side of the seal towards the low pressure side. Bending of the bristles in this way reduces the effective radial length of the bristles and so reduces the contact between the bristle tips and the shaft. In turn this reduces the sealing efficiency. Additionally flexing of the bristles can clamp the bristles against each other and against a backing member of the seal. Friction between the bristles and the backing member stiffens the bristles and prevents free movement of the bristles so that they no longer follow the shaft. Excursions of the rotating shaft are therefore not accommodated by the bristles. This leads to clearance gaps opening up between the bristle tips and shaft, and at the other extreme increased wear of the bristles, both of which reduce the seal performance.
An alternative type of seal that has been proposed for gas turbine engines, and also other applications, is a leaf seal. Leaf seals, which are sometimes called foil seals, comprise a number of thin resilient yet flexible strips which are densely packed together with the strips substantially aligned in the sealing direction. One end of the strips is held within a housing whilst the other, distil end of the strip extends across the seal gap and rests on the shaft. Such a seal design is described in European Patent Application EP 0,391,676. As described the seal elements, comprising the resilient strips in this type of seal are axially stiff and do not tend to bend in the axial, sealing, direction. This type of seal thereby alleviates some of the problems described above in relation to brush seals.
In EP 0,391,676 the resilient strips are described as being packed essentially solidly. This will prevent or restrict the radial movement of the strips. Consequently the seal described is also stiff in a radial direction. Problems can therefore occur during radial excursions of the shaft into the strips. The radial stiffness of the seal reacts against the radial movement of the shaft, causing high tip loads and increased wear of the strips.
It is therefore desirable to provide an improved seal design that alleviates the above problems and/or offers improvements generally.